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KT Eri (Nova Eridanus 2009)

KT Eri is a very fast classical nova that was discovered by Itagaki at a CCD magnitude of 8.1 on November 25, 2009. Like many novae, it was discovered after reaching its peak brightness, but pre-discovery images were later found, giving its peak magnitude at 5.4 on November 14 (Hounsell et al. 2010). It declined to about magnitude 11.5 by mid-February 2010, but remained fairly constant throughout the rest of the 2009-2010 season, with irregular flickering of about half a magnitude. KT Eri had declined to 14th magnitude by late 2010, and to around 15th by early 2011.

This nova has several claims to fame, the first being the fact that the peak was later confirmed partially through CCD observations by the Solar Mass Ejection Imager (SMEI). This instrument is part of the Coriolissatellite, a mission launched in 2003 into a sun-synchronous orbit 840 km above the earth. The satellite can resolve brightness changes in stars down to about 8th magnitude; astronomers therefore expected that the satellite might be useful in detecting bright novae that might otherwise escape detection, especially stars in the same general region of the sky as the sun. The hope is that SMEI might not only detect previously unknown novae (and capture peak data on novae whose maxima were missed by visual and CCD ground-based observers), but also provide constraints on the rate of novae in our galaxy. As of the end of 2010, three classical novae (KT Eri, V598 Pup, and V1280 Sco) and one recurrent nova (RS Oph) had been observed with enough detail to complete light curves solely using SMEI data. It is estimated that perhaps five novae a year might be discovered by SMEI in the future (Hounsell et al. 2010).

AAVSO observations of the nova now number over 24,000. Some of these were in response to a January 2010 AAVSO request for observations, posted at the request of Dr. Julian Osborne of the University of Leicester, in order to support an x-ray observation campaign using the Swift satellite. Those observations were cited in the resulting paper (Schwarz et al. 2011). Osborne and his colleagues were especially interested in KT Eridani because it was the second nova, after the recurrent nova RS Oph, found to have a roughly 35 second variation in the so-called super soft source (SSS) x-ray emission (Beardmore et al. 2010). The SSS emission is thought to be associated with nuclear reactions on the surface of the white dwarf; in the case of KT Eri the SSS emission began in a way similar to the recurrent nova LMC 2009a. There were other similarities in the optical light curve (especially that derived from SMEI data) and spectroscopy, not only to LNC 2009a and other recurrent novae, but in particular to a subclass of recurrent novae that are thought to be the progenitors to Type Ia supernovae. As a result, astronomers searched for previous outbursts of KT Eri in available Harvard College Observatory photographic plate data (Jurdana-Sepic et al. 2001). A periodicity of 737 days was found during the quiet phase of the system, as well as another period at 376 days, suggesting that the secondary (mass-donating) star in the system is a red giant star rather than a main sequence star. Combined with the 9 magnitude outburst KT Eri displayed in 2009, the system appears to have more in common with recurrent novae than very fast classical novae. The study did not find any evidence for previous outbursts between 1883-1952, but another study is examining data from the intervening decades. It is therefore still possible that KT Eri could be recurrent on timescales of a half a century to a few centuries. Perhaps papers at the AAVSO’s tercentennial will include observations of this star’s next outburst.